The provision of timing information is an essential feature of wireless communication systems which allows to ensure synchronicity among the distributed system components. In almost every wireless communication system the timing information is obtained from a transmit signal analyzed on a receiving side.
In the following, an approach for extracting timing information from an input signal is exemplarily described for wireless communication systems operating in accordance with Orthogonal Frequency Division Multiplexing (OFDM).
OFDM is a multicarrier modulation scheme which is especially suited for highly frequency-selective transmission channels such as typical channels for mobile communication systems or for high-rate wired transmission via copper lines. Highly frequency-selective channels are characterized by impulse responses which are substantially longer than one sample interval. Therefore, each received sample in a digital base band domain is a superposition of several transmit samples weighted by the respective channel coefficients. This means that highly frequency-selective channels are subject to intersample interference.
The principle of OFDM to combat intersymbol interference is to divide the total channel bandwidth into substantially smaller portions, i.e. subchannels. A sequence of samples to be transmitted is combined to a single OFDM symbol and transmitted in parallel on these subchannels. A single OFDM symbol thus uses all of the subchannels in parallel. In accordance with OFDM, transmitted subchannel signals are orthogonal to each other. Since the duration of one OFDM symbol is much longer than the sample interval, intersymbol interference is strongly reduced.
To further reduce intersymbol interference, usually a guard interval is introduced between two OFDM symbols which are to be consecutively transmitted. If the length of the guard interval exceeds the length of the channel impulse response, there is no residual intersymbol interference. Furthermore, if the guard interval is constituted by a repeated signal portion, e.g. a cyclic prefix, a very simple equalization of the frequency-selective channels in the frequency domain is possible.
However, since the use of a guard interval leads to additional transmission overhead, the length of the guard interval is usually chosen such that the intersymbol interference is not totally cancelled. Rather, only the main contributions of typical channels are accommodated in the guard interval and residual intersymbol interference is tolerated.
An OFDM receiver has to perform synchronization prior to demodulation of the subcarriers. The task during synchronization is to find an optimal timing for minimizing the effects of intersymbol interference. Therefore, timing information allowing to find out the optimal timing instant for synchronization purposes has to be provided.
Several synchronization approaches are known in the art. Most of these approaches are based on the exploitation of repeated signal portions within a transmit signal. Usually, the repeated signal portions are located at predefined locations of a so-called repetition preamble. An example for synchronization of OFDM systems based on a repetition preamble is described in M. Speth, F. Classen and H. Meyr, Frame Synchronization of OFDM Systems in Frequency Selective Fading Channels, VTC '97, Phoenix.
In an OFDM receiver the received sample stream is processed in order to recognize the repeated signal portion. Several metrics to detect repetition preambles for synchronization purposes are exemplarily described in S. Müller-Weinfurtner, On the Optimality of Metrics for Coarse Frame Synchronization in OFDM: A Comparison, PIMRC '98, Boston. These metrics make use only of the cyclic nature of the repeated signal portion but not of the actual content thereof.
A synchronization method which actually exploits the content of a repeated signal structure is known from R. van Nee, R. Prasad, OFDM for wireless multimedia communications, Artech House, 2000. According to this synchronization method, a matched-filter approach is pursued to achieve optimal timing synchronization for OFDM in a multipath environment. During matched filtering a special OFDM training signal derived from a transmit signal portion is used for which the data content is known to the receiver. In the matched filter, a received transmit signal is correlated with the known OFDM training signal. The resulting matched filter output signal comprises correlation peaks from which both timing information and frequency offset information can be derived.
The filter tap values used during matched filtering are obtained from training values comprised within the known OFDM training signal. According to a first approach, the filter tap values equal the transmitted training values. According to a second approach, the filter tap values are derived from the training values by means of quantization. Quantization reduces the overall complexity of the matched filter since the multiplications necessary during the correlation operations can thus be reduced to additions.
By means of quantization, each of the real part and the imaginary part of the training values is mapped separately on the nearest integer from the set of {−1, 0, 1}. The quantization is thus performed individually for the real and imaginary parts. This means that after quantization the filter tap values will usually also comprise a real and an imaginary part each. This leads to four additions per correlation operation. The number of zeros in the resulting set of quantized values is fixed depending on the individual training values comprised within the training signal.
There is a need for a method and a device for providing timing information for a received transmit signal which allow to derive the timing information in an efficient and flexible manner.